1. Field of the Invention
The present invention relates to a throttle body to be used for adjusting output of an engine, and more specifically to a throttle body for traction control provided with a main throttle valve which is opened and closed through operation of an accelerator and with a secondary throttle valve which is opened and closed by means of a motor.
2. Description of the Prior Art
A throttle body for traction control provided with a main throttle valve and a secondary throttle valve has been often used, and a structure which constitutes the basis of the present invention is disclosed in Japanese Laid-Open Utility Model Publication No. 64-46443. The structure includes a speed reducer interposed between a shaft for a secondary throttle valve and an output shaft of a motor for opening and closing the secondary throttle valve, so that a requisite torque for opening and closing the secondary throttle valve is obtainable through the speed reducer. To assure fail-safe operation, the secondary throttle valve is required to be opened when the motor for opening and closing the secondary throttle valve is in trouble. When the motor is in trouble, closing of a throttle bore by the secondary throttle valve would cause stoppage of the engine, while opening thereof by the secondary throttle valve permits adjustment of the engine output through a main throttle valve. For this purpose, a return spring is mounted on the secondary throttle shaft so as to urge the secondary throttle valve toward its fully opened position. Then, the motor for opening and closing the secondary throttle valve is required to produce a torque large enough to close the secondary throttle valve against the biasing force of the return spring. The technique disclosed in the above Japanese Laid-Open Utility Model Publication No. 64-46443 employs the speed reducer, which permits a relatively small motor to be used as the motor for opening and closing the secondary throttle valve.
This technique allows the motor for opening and closing the secondary throttle valve to be made smaller, but requires provision of the speed reducer. This results in addition of a manufacturing cost of the speed reducer to the whole cost of manufacturing the throttle body. Furthermore, an additional space for installing the speed reducer is necessary, which will be disadvantageous to make the throttle body smaller.
Japanese Laid-Open Patent Publication No. 3-290042 discloses another structure in which a secondary throttle shaft and an output shaft of a motor for opening and closing the secondary throttle valve are directly connected by means of a coupling which can transmit torque therebetween, with no speed reducer interposed between the shafts. This structure is effective to solve the above problem. In this structure with no speed reducer, however, the motor for opening and closing the secondary throttle valve is required to close the secondary throttle valve directly against the biasing force of the return spring, so that the motor must produce a larger torque. Thus, this structure requires provision of a larger motor.